My invention relates to a frequency indicating circuit, and particularly to a frequency indicating circuit for producing signals whose characteristics show the relative frequency of two signals.
An indication of the relative frequency of two signals is needed in many electronic applications. In particular, this indication is needed where an unknown signal frequency is to be corrected or changed to conform to a reference signal frequency.
Accordingly, a primary object of my invention is to provide a new and improved circuit for indicating the relative frequencies of two signals.
Voltage controlled oscillators are used extensively in phase lock loop circuits to generate a frequency which can be varied or controlled, but which must have the basic stability of a reference frequency oscillator. Frequently, the controlled oscillator frequency is so different or so far removed from the reference oscillator frequency that the phase lock loop circuit does not have the pull-in range or ability to achieve control. As a result, the phase lock loop circuit hunts or varies continuously without achieving the desired control.
Accordingly, another object of my invention is to provide a new and improved frequency indicating circuit that produces a signal indicative of any frequency difference between two signals.
Another object of my invention is to provide a new and improved frequency indicating circuit that produces a signal which can be used with a phase lock loop circuit to acheive frequency synchronization or control, after which the phase lock loop circuit can achieve phase synchronization.
The control signal used to achieve frequency synchronization in a phase lock loop circuit may be in a circuit carrying modulation or other information which should not be changed or distorted. Accordingly, another object of my invention is to provide a frequency indicating circuit which produces a control signal that is relatively free from noise.